Why did Imam Khomeini think that principle of enjoining good and forbidding evil has been abandoned?

The first part of the principle consists of promoting goodness and all divine virtues among the societies while the latter part refers to undertaking efforts for rooting out bad deeds and any types of carnal desires, lusts and corruption.

The principle is one of the most important Islamic duties, indeed it is the noblest and most sublime. This is the task of the Prophets and Messengers (peace be upon them all), as Allaah says (interpretation of the meaning): 

“Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allaah after the (coming of) Messengers” [al-Nisaa’ 4:165] 

“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are the successful” [Aal ‘Imraan 3:104] 

God, the Almighty has made the Muslim ummah the best nation ever raised up for mankind, in order to do this important task, as Allaah says (interpretation of the meaning):  

“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al-Munkar (polytheism, disbelief and all that Islam has forbidden), and you believe in Allaah” [Aal ‘Imraan 3:110] 

If the ummah fails to do its duty of enjoining what is good and forbidding what is evil, wrongdoing and corruption will spread throughout the ummah, and it will deserve the curse of Allaah. For Allaah cursed those among the Children of Israel who disbelieved because they failed in this important duty. Allaah says (interpretation of the meaning): 

Imam Khomeini wanted to revive this principle to be revived in all its aspects.